In the galvanizing art, practitioners have long combated the formation of dross on the tops of the vessels of molten zinc. Dross is a skim which forms on the top of a bath of molten metal and, in the case of zinc, consists of a mush-like mixture of iron, aluminum and zinc intermetallic particles, oxide skims and liquid zinc alloy. The dross consists of about 96% zinc and 2% each of aluminum and iron. The dross is frequently removed for processing to prevent fouling the surface of the galvanizing strip product and to recover the zinc retained in the dross.
Even most contemporary procedures for removing dross in galvanizing and other molten metal processes are relatively crude, generally labor-intensive and dangerous for the operators of the removal apparatus. The removal process generally consists of having an operator use a long-handled paddle or rake to skim the surface of the molten bath. The dross is then scooped into a nearby mold.
Some of the apparatus and processes proposed in the past have numerous moving parts which are prone to breakdowns in the hostile environment of the galvanizing plant. Examples of apparatus having numerous parts are the mechanical skimmers disclosed in U.S. Pat. No. 3,941,360 to Blank, U.S. Pat. No. 4,165,866 to Hetra, et al., and U.S. Pat. No. 4,007,119 to Antonevich. Mechanical skimmers are still not used in many facilities because of various shortcomings in their design and because the process does not lend itself well to automation.
Various prior art workers have observed that, while galvanizing dross typically contains considerable amounts of zinc, the zinc tends to drain back into the vessel if the dross is contained in a temporary enclosure. Typically the temporary enclosure has perforations or other holes in it to permit the zinc to drain back into the main body of molten metal. An illustration of such an enclosure similar to that sometimes used in the galvanizing art is the one shown by Reding in U.S. Pat. No. 3,843,355, said to be specifically for the magnesium purifying art. See also the temporary enclosure or "flotation box" of Tyler, et al. shown in U.S. Patent No. 4,386,958.
In U.S. Patent No. 4,003,559, Kuwano, et al. note that a common practice in the art has been to agitate the dross manually to hasten the coalescing of the zinc droplets or particles, to drain zinc from the dross. We have found that since the manual agitation of the dross is difficult and dangerous to perform, it is seldom performed efficiently, resulting in a high concentration of useable zinc remaining in the dross for recovery in a remote location rather than directly into the molten bath. Kuwano, et al. go on to suggest the use of a plurality of "scrape plates" for mechanical, rather than manual, agitation of the dross. The scrape plates are disposed within a container which is separate from the main metal bath. The container has perforations or holes for draining coalesced metal.